I am sorry, but I just cannot find this information anywhere, and our
archivist doesn't know either. For the attribute source in corpname, if it
is the LOC Online Catalog, what is the code I should use for source?

This actually illustrates the hardest part of creating a finding aid, at
least for me. The online docs do a great job of explaining what the
elements are, but gloss over what the allowable attributes are....
/end non-archivist opinion